The present invention concerns scanning microscopes and in particular the mechanisms that they employ for varying the position of the target spot in the object region.
A type of scanning microscope that illustrates aspects of scanning-microscope operation is the confocal microscope depicted in FIG. 1. In that drawing, an objective 12, typically (although not necessarily) of the type employed in a conventional microscope, images an object plane 14 into an image plane 16 in the ordinary manner. In a conventional microscope, of course, the entire viewed portion of the image plane 16 is continually illuminated, and the objective optics must simultaneously keep all parts of the viewed object plane 14 in focus; the conventional microscope can be thought of as processing all picture elements ("pixels") in parallel. As those skilled in the art recognize, this necessitates many compromises that result in optical noise, distortion, and limited resolution. Also, the light from the illumination source must be shared among all regions of the object plane, so any single small part of the viewed object receives only a small fraction of the source's emitted light power.
Microscopes of the scanning type, and in particular confocal microscopes, reduce these conventional-microscope limitations. The light source, typically a laser 18, transmits light through a path that includes a scan lens 20, which focuses nearly the entire laser output into a small point in the image plane, which in turn is conjugate to a correspondingly small target spot in the object plane 14. Consequently, the laser illuminates only a small target spot in the object plane 14 at any one time. Nearly the entire laser output power is therefore delivered to a single target spot, which, because of the scan lens 20 and another, detector lens 22 and certain other elements described below, is also conjugate to an entrance aperture 24 of a photodetector 26.
More specifically, the output of the laser 18 travels through a beam expander 28 having an internal focal plane 30 that, like the detector's entrance aperture, is also conjugate to the single spot in the image plane 16, and the laser beam thus expanded is directed by (in the illustrated example) two mirrors 32 and 34 to a dichroic mirror 36, which passes the laser light through a scanner assembly 38 to be described below. The scanner assembly 38 forwards the light through the scan lens 20 and the objective 12. The reflected light returns through the objective 12, the scan lens 20, and the scanner assembly; i.e., the incoming- and reflected-light paths share a common path segment. The two paths branch at the dichroic mirror 36, by which the returning light is reflected through the detector lens 22 to the detector 26.
The purpose of the scanner 38 is to deflect both the incoming light from the laser 18 and the reflected light from the object plane 14 and thereby move the spot in the image plane 16--and thus in the object plane 14--to which both the beam-expander focal point 30 and the detector aperture 24 are conjugate. The scanner typically moves this point in a raster-scan fashion, and a raster-scan display 40 operated in synchronism with the scanner 38 displays the resulting detector output.
Such an arrangement has significant performance advantages, as those skilled in the art recognize. Among these, for instance, is that the detector aperture 24 can function as a pinhole, providing spatial filtering to improve resolution, as can a corresponding aperture placed in the beam-expander focal plane 30. Additionally, since only a very small part of the object plane is illuminated at any one time, very little light from other object-plane locations is available to be imaged, because of optical-system distortions, to the detector pinhole and thus act as noise. Another advantage is the considerable light efficiency that results: the power required to illuminate an object in a scanning-type microscope is a very small fraction of that required to obtain the same level of illumination in a conventional microscope. Moreover, because such an illumination approach makes it practical to achieve a high instantaneous light intensity at the target spot, certain desirable imaging techniques, such as those that employ fluorescence, tend to be more practical than they would otherwise be.
By employing high-numerical-aperture objectives, confocal microscopes are particularly well adapted to forming three-dimensional images of semi-transparent objects such as biological samples. The microscope takes a series of two-dimensional "slices," each taken at a different depth into the sample. The slices thus taken can be viewed sequentially or processed by computer to generate slices through the sample at different angles. By employing similar techniques, high-resolution images of non-flat opaque objects can also be taken.
However, certain difficulties arise because of the scanner mechanism. FIG. 2 depicts a typical scanner mechanism. It includes a first deflection mechanism in the form of a galvanometrically driven mirror 42 that so pivots as to cause the target spot to move in directions parallel to an axis that can be called the x axis. In doing so, mirror 42 deflects the laser beam to a similarly driven second mirror 44, which causes movement of the target spot in directions parallel to an orthogonal, y axis. In the illustrated scanner mechanism, mirror 44 pivots about an axis that extends through its center, while mirror 42 pivots in a "paddle" fashion about an axis 46 spaced from the mirror surface. The result of this arrangement is that, although the angle at which light from the laser source is reflected from mirror 42 varies, the location at which that returning beam hits mirror 44 does not change appreciably. Consequently, mirror 44, which must be pivoted many times as quickly as mirror 42, can be kept relatively small.
Despite this small size, it is difficult to obtain the speed necessary to meet the rather severe requirements to which confocal microscopes can be subject. Because of the multiple scanning mentioned above, the acquisition time for a complete image can be objectionably long unless each raster scan is extremely fast. Moreover, confocal microscopes tend to be quite expensive, and it is desirable to mitigate this tendency to a degree by employing as few specialized parts as possible. It is thus beneficial to make the mechanical scanning compatible with television scanning rates, which are quite high. This will be particularly true when compatibility with high-definition television becomes desirable.
These difficulties apply not only to confocal microscopes, which provide scanning in the paths both from the source and to the detector, but also to some other types of scanning microscopes, which provide scanning in only one of the paths.